[whatwg/dom] Add declarative Shadow DOM features (#892)

The explainer for this feature is here: https://github.com/mfreed7/declarative-shadow-dom/blob/master/README.md
The issue discussion is here: https://github.com/whatwg/dom/issues/831
There is a corresponding [Pull Request](https://github.com/whatwg/html/pull/5465) for the HTML spec that goes along with this PR.

Note, due to some local branch issues, this is a duplicate of [PR 858](https://github.com/whatwg/dom/pull/858). Please see that PR for more history and review comments here. In particular, the last comment there [is from me](https://github.com/whatwg/dom/pull/858#issuecomment-668875565) - it is my belief that I've addressed all of the outstanding comments.

- [ ] At least two implementers are interested (and none opposed):
   * I'm working to get this implemented in Chromium, tracking bug [here](https://crbug.com/1042130).
   * Other implementers have at least not been negative on the [discussion thread](https://github.com/whatwg/dom/issues/831). Not explicitly positive (yet).

- [X] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * https://wpt.fyi/results/shadow-dom/declarative?label=master&label=experimental&aligned&q=declarative

- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/master/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chrome: https://crbug.com/1042130
   * Firefox: None yet
   * Safari: None yet
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/892

-- Commit Summary --

  * Added "is declarative shadow root"
  * Added existing shadow root support
  * Add getInnerHTML()
  * Pulled out "attach a shadow root" algorithm
  * Add note about multiple declarative roots
  * Remove link to "declarative shadow root"
  * Modified clone() to allow cloning templates with declarative SD
  * Better logic for detecting template
  * Two quick fixes
  * Cleanup
  * Only template contents
  * Fix HTML link
  * Addressed annevk's comments

-- File Changes --

    M dom.bs (114)

-- Patch Links --

https://github.com/whatwg/dom/pull/892.patch
https://github.com/whatwg/dom/pull/892.diff

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